DCS (Dichloro-Silane (SiH2C2))-based WSix films have become desirable in semiconductor fabrication processes due to their low resistance properties. Such films are especially amenable for use in the formation of gate/bit line interfaces for transistors. However, traditional approaches for fabrication of such films, such as those disclosed in U.S. Pat. No. 5,786,027, to Rolfson, and U.S. Pat. No. 5,425,392, to Thakur et al., result in the abnormal growth of polysilicon in crystallized regions of the wafer, which in turn, decreases production yield.
In the conventional approach, a polysilicon seed layer is initially deposited on an underlying substrate, for example, above an active region of a transistor. Transistor gate layers and/or bit lines are provided above the polysilicon layer through the layering of a DCS seed using DCS gas deposition. Next, the silicon-rich DCS layer undergoes nucleation by exposure to WSix, followed by bulk deposition of Tungsten and polysilicon layers. This is followed by a DCS post flush, that serves to eliminate impurities such as Cl and F that remain as a result of nucleation and bulk deposition the WSix layer. A mono-silane (SiH4) post flush is then performed to eliminate high stress that exists between the Tungsten and polysilicon layers, which can lead to delamination of the layers. In general, the longer the mono-silane flush is performed, the greater the improvement in stress reduction. Following this, gate and/or bit line patterns are provided over the resulting structure, and subsequent layers are formed.
During formation of the polysilicon layer, the substrate is heated to the dissolution temperature of the DCS gas, i.e. 620 C, in order to reduce sheet resistance in the layer. However, this causes the underlying polysilicon layer to become crystallized, which in turn, leads to its abnormal growth. This, in turn, can cause stress fractures that have deleterious effects, i.e. surface cracking, during subsequent stages of the process.
Additionally, while the mono-silane flush serves to reduce stress between the WSix and polysilicon layers, it also causes further infusion of Si into the underlying polysilicon layer, which can likewise lead to its abnormal growth, along with stated adverse effects on the process and device yield.
To address the limitations of conventional approaches, the method of the present invention provides for the fabrication DCS-based films, while mitigating or eliminating the effects of abnormal polysilicon growth. A first approach conducts the deposition of the underlying polysilicon layer at a temperature that substantially avoids crystallization of the polysilicon. A second approach reduces the exposure level (for example time period and/or concentration level) for the mono-silane SiH4 post flush, so as to avoid infusion of silicon into the underlying polysilicon layer, and resulting abnormal growth of the polysilicon.
In a first embodiment, the present invention is directed to a method of forming a double-layered semiconductor film. A polysilicon layer is provided on an underlying substrate in a diffusion process conducted at a first temperature that substantially avoids crystallization of the polysilicon. The temperature of the polysilicon layer is raised to a second temperature. The polysilicon layer is flushed with a first flush material to provide a transition layer. The polysilicon layer is next flushed with a second flush material to provide a second material layer over the polysilicon layer, the transition layer providing adherence characteristics between the second material layer and the polysilicon layer. A combination of the first flush material and the second flush material are provided to deposit a bulk second material layer on the transition layer. The bulk second material layer is flushed with the second flush material to remove impurities. The bulk second material layer is then flushed with the first flush material to mitigate stress between the polysilicon layer and second material layer.
Flushing of the bulk second material layer with the first flush material is preferably limited in time duration so as to substantially avoid abnormal growth of the underlying polysilicon layer. Alternatively, flushing of the bulk second material layer with the first flush material can be limited in concentration to substantially avoid abnormal growth of the underlying polysilicon layer.
The first temperature is preferably equal to or less than 530 Celsius. The second temperature is preferably equal to or greater than 620 Celsius, in order to reduce resistivity in the resulting double-layered semiconductor film.
The polysilicon layer is preferably pre-clean following its deposit, which deposit can be conducted at atmospheric pressure.
The second material layer preferably comprises tungsten silicide WSix.
Following flushing of the polysilicon layer with a second flush material, nucleation of the second material layer can be performed to reduce grain size of the second material layer. Following flushing of the bulk second material layer with the second flush material, nucleation of the bulk second material layer can be provided to reduce grain size of a top portion of the bulk second material layer. The first flush material may comprise silane SiH4 and the second flush material may comprise di-chloro silane (DCS) SiH2Cl2 and tungsten fluoride WF6.
In a second aspect, the present invention comprises a method of forming a double-layered semiconductor film. A polysilicon layer is provided on an underlying substrate at a first temperature. The temperature of the polysilicon layer is raised to a second temperature. The polysilicon layer is flushed with a first flush material to provide a transition layer. The polysilicon layer is next flushed with a second flush material to provide a second material layer over the polysilicon layer, the transition layer providing adherence characteristics between the second material layer and the polysilicon layer. A combination of the first flush material and the second flush material are provided on the resulting structure to deposit a bulk second material layer on the transition layer. The bulk second material layer is flushed with the second flush material to remove impurities. The bulk second material layer is flushed with the first flush material to mitigate stress between the polysilicon layer and second material layer, the exposure of first flush material being limited so as to substantially avoid abnormal growth of the underlying polysilicon layer.
In a preferred embodiment, the step of providing a polysilicon layer is performed in a diffusion process conducted at the first temperature so as to substantially avoid crystallization of the polysilicon.
The exposure of first flush material is preferably limited in time duration and/or concentration so as to substantially avoid abnormal growth of the underlying polysilicon layer.